A medical MRI device uses a phenomenon called nuclear magnetic resonance that occurs when a spherical or an elliptically spherical imaging space in which a static magnetic field is uniformly distributed is irradiated with an electromagnetic wave so as to acquire tomographic images of an examinee. Unless uniformity of the magnetic field in the imaging space is satisfactory in the MRI device, the images may distort or their shades may become irregular. Thus, the magnetic field in the imaging space needs to have uniformity on the order of several ppm. In addition, the magnetic field of the MRI device has been strengthened so as to improve the S/N ratio of images and acquire clear images.
MRI devices are mainly categorized as a tunnel-type and an open-type based on their shapes. The latter type allows the space around the examinee to be wider than the former type. That is, since the latter type has a large degree of openness than the former type, claustrophobia of the examinee can be reduced. In addition, the latter type is superior to the former type in accessibility to the examinee. A typical open-type MRI device has a structure in which a pair of main coils that generates main magnetomotive force and a pair of coil groups or a pair of magnetic substances that adjusts uniformity of the magnetic field are disposed in a pair of vertically divided opposite magnetic poles.
As a related art of the present technical field, JP 2003-061931 A (PTL 1) is known. The abstract of the publication describes that “laminates 24a, 24b of directional silicon steel plates are disposed on opposite surfaces 32 of annular protrusion portions 20a, 20b so that the surfaces of the steel plates become nearly in parallel with the opposite surfaces 32. The course of magnetic fluxes generated by superconducting coils 26a, 26b is changed to the direction represented by an arrow 33 at the laminates 24a, 24b. As a result, the magnetic field strength of an outer circumferential portion (A portion) of a uniform magnetic field region 12 increases.” In addition, paragraph [0056] of the publication describes that “according to the present embodiment, a laminate 48b of a lower directional silicon steel plate is disposed and secured on an inner circumferential side of the lower annular protrusion portion 20b.”